Photocatalysts such as titanium oxide have recently become extensively utilized. Activity excited by photoenergy of photocatalysts can be utilized to decompose various harmful substances or to hydrophilify a surface of a member with a photocatalyst particle-containing surface layer formed thereon, whereby fouling deposited on the surface by water can easily be washed away.
A method in which the layer is formed utilizing a binder component having corrosion resistance to a photocatalyst and is brought into close contact with a surface of a substrate, is known as a method for the formation of a photocatalyst particle-containing layer on a surface of a substrate (for example, JP H07(1995)-171408A (PTL 1)).
Various types of binders have been proposed in these methods. Examples thereof include fluororesins (for example, JP H07(1995)-171408A (PTL 1)), silicones (for example, JP 2005-161204A (PTL 2)), silica particles (for example, JP 2008-264747A (PTL 3)), zirconium compounds (for example, WO 99/28393 (PTL 4)), aluminum compounds (for example, JP 2009-39687A (PTL 5)).
In the construction of the photocatalyst layer formed on a surface of a substrate, when the substrate is an organic material, there is a possibility that the organic material is decomposed or deteriorated by photocatalytic activity of the photocatalyst. To cope with this problem, a technique is known in which an adhesive layer such as a silicone-modified resin is provided between the photocatalyst layer and the substrate to protect the substrate as the substrate from a deterioration caused by photocatalytic action (WO 97/00134 (PTL 6)). In this prior art, an example in which the amount of the photocatalyst exceeds 20% by weight was disclosed. Further, it is described that the decomposition or deterioration of the substrate could be effectively prevented.
A proposal has also been made in which an intermediate layer comprising a silicone-modified resin and an organic antimold agent is provided between a photocatalyst layer and a substrate to prevent the decomposition and deterioration of the substrate (JP 2008-272718A (PTL 7)).
Various proposals on a technique in which NOx is decomposed utilizing photocatalysts have been made (for example, JP H01(1998)-218622A (PTL 8), JP 2001-162176A (PTL 9), JP 2008-264747A (PTL 3))
What is important of NOx decomposition is to efficiently decompose NOx and, at the same time, to suppress the production of harmful intermediate products such as NO2. In decomposing NOx by photocatalysts, the development of a technique that can suppress the production of harmful intermediate products is desired.
JP 2009-270040A (PTL 11) is a prior art which discloses a combination of photocatalyst with zirconium compounds. PTL 11 discloses a photocatalyst coating liquid comprising photocatalytic titanium oxide, zirconia particles where D50 is 1 to 20 nm, and carboxylic acid. The zirconia particles are included in order to enhance adhesion of photocatalyst layer. When titanium oxide is 100 parts by mass, zirconia particles are not less than 25 parts by mass and not more than 100 parts by mass. However, the patent publication does not disclose addition of silica particles. The patent publication discloses evaluation of aldehyde decomposition capability, but does not disclose NOx decomposition capability and weather resistance.
WO 97/00134 (PTL 6) discloses a photocatalyst layer comprising titanium oxide, silica and zirconium oxide sol. (for example, Example 27) PTL 6 disclose discloses zirconium tetrabutoxide as a zirconium oxide sol, which is dried by heating. The diameter of the zirconia particles after dried by heating under the conditions disclosed in PTL 6 is considered to be several μm.
Further, JP 2009-39687A (PTL 5) discloses a photocatalyst-coated body comprising photocatalyst particles, silica and zirconium acetate. In PTL 5, the compositions including zirconium acetate are cured at room temperature. Accordingly, it is considered that zirconium compounds that are produced after the cure are not in the form of particles. PTL 5 also discloses evaluation of aldehyde decomposition capability of the photocatalyst-coated body, but does not disclose NOx decomposition capability and weather resistance. Furthermore, in PTL 5, the addition amount of silica is less than 30 parts by mass.
WO 98/015600[PTL 12] discloses a photocatalyst layer comprising photocatalysts, zirconium compounds and/or tin compounds, silica compounds. According to PTL 12, the zirconia compounds are included from the viewpoint of imparting alkaline resistance. According to PTL 12, the zirconium tetrabutoxide as a zirconium oxide sol is dried by heating in order to obtain zirconium compounds. Accordingly, it is considered that monoclinic zirconia is produced under the conditions according to PTL 12 and that the particle diameter thereof is several μm.